1. Field of the Invention
The invention relates to an organic electroluminescent device (hereinafter referred to as organic EL device, electroluminescent device, or EL device) for emitting light by converting electric energy to light.
2. Description of the Related Art
Investigation and development of various kinds of display devices is flourishing and organic electroluminescent (organic EL) devices in particular have drawn attention as up-and-coming display devices, since bright light emission can be achieved at low voltage.
An organic electroluminescent device comprises a pair of counter electrodes sandwiching a light-emitting layer, or plural organic layers including a light-emitting layer, and emitting light based on electroluminescence from an exciton, produced by recombination in the light-emitting layer of an electron injected from a cathode and a hole injected from an anode, or based on electroluminescent from an exciton of another molecule produced by energy transfer from such an exciton.
However, organic electroluminescent devices need further improved light emitting efficiency and durability particularly, in blue- and green-emission.
To address the above-mentioned issues, in order to improve the operational efficiency of a device, techniques for forming a carrier flow rate suppressing layer for adjusting the balance of the carrier recombination have been disclosed.
For example, an organic EL device comprising a hole flow rate-suppressing layer having a smaller ionization potential (hereinafter sometimes referred to as IP) than a hole-transporting layer provided between the hole-transporting layer, and the light-emitting layer is disclosed (e.g. Japanese Patent Application Laid-Open (JP-A) No. 2000-243574).
Also, an organic electroluminescent device comprising an interface layer (a hole flow rate-suppressing layer) with a thickness of 0.1 to 5 nm and having higher IP than a hole-transporting layer and higher energy band gap than a light-emitting layer between the hole-transporting layer and light-emitting layer is disclosed (e.g. reference to JP-A No. 2003-123984)
That is, the correlations of IP among the hole flow rate-suppressing layer and respective organic layers are classified as follows: as shown in FIG. 1A and FIG. 1B, (IP of hole-transporting layer)>(IP of hole flow rate-suppressing layer)<(IP of light-emitting layer) in the case of JP-A No. 2000-243574; and (IP of hole-transporting layer)<(IP of hole flow rate-suppressing layer)>(IP of light-emitting layer) in the case of JP-A No. 2003-123984.
However, formation of the layer for suppressing the injection of the hole involves a problem that the charge stagnates at the interface of (hole-transporting layer/hole flow rate-suppressing layer) or at the interface of (hole flow rate-suppressing layer/light emitting layer) and this accelerates the deterioration of the materials and thus these devices have insufficient operational durability.